NO. 1395. CAMBRIAN BRACIIIOPODA—WALCOTT. 247 



The ventral valve slijihtly flattened at the cardinal extremities, rising 

 toward the center with a convex triangular swelling, broadening fioiu 

 the narrow beak to the front; beak small, rounded, and extending 

 slightly beyond the hinge line. Dor.sal valve flattened at the cardinal 

 extremities, with well-marked rounded ridges rising between them, 

 and a rather broad, well-defined median sinus; beak very small, slightly 

 encroaching upon the hinge line. 



Surface marked by flne, regular, radiating striae, between which 

 one or more faint intermediate strige are sometimes visible; under 

 favorable conditions very fine concentric stride can be seen, and there 

 are also usually present more or less distinctly marked lines of growth. 



Very little is known of the interior of the ventral valve. Two frag- 

 ments of the posterior portion show a broad delthyrium, strong teeth, 

 and a broad umbonal cavity opposite the delthyrium. In the ventral 

 valve the area is narrow and divided by a broad delthyrium, which 

 has a strong cardinal process that extends forward nearly three times 

 the length of the area. Anteriorlv, it rests on a broad ridge that 

 extends forward, gradually broadening out and disappearing at the 

 anterior margin of the shell. 



In my former description « I stated that "the generic character of 

 this species has not been fully ascertained, but the material from the 

 Yellowstone National Park and specimens from the typical locality at 

 Red Wing, Minnesota, lead me to think that this can not be referred 

 to the genus Blllingnella. It appears to be an Orthis of the Plector- 

 this group of Hall and Clarke." A more thorough study of this spe- 

 cies and the forms referred to BiUhigseIJa leads me to place it in a 

 subgenus of Billingsella on account of its surface character, the char- 

 acter of the delthyrium in the two valves, and the strong cardinal 

 process and median ridge in the dorsal valve. 



A comparison of the specimens of this species from a typical locality 

 at Red Wing, Minnesota, with those from the Yellowstone National 

 Park, shows the two shells to l^e speciflcally identical as far as the 

 comparison of casts in sandstone can be made with well-preserved 

 shells on the surface of limestone. This is the only species of this type 

 known to me in the Cambi'ian fauna. 



Formation and locality.— Vp-pev Cambrian. St. Croix sandstone, 

 Red Wing, Minnesota. Upper Gallatin limestone, north side of Elk 

 Pass, between Buffalo and Slough Creeks, Yellowstone National Park, 



Montana. 



NISUSIA, new genus. 



Form subquadrate to transversely semioval. Shell substance fibrous; 

 surface with narrow, radiating ribs that support irregularly distributed 

 spines on their crests. 



«Mong. XXXII, U. S. Geol. Siir., p. 453. 



